The PasteAsPlainText script is for copying text from an outside application, such as Word. It relies on processing the external system Clipboard to get the text into InDesign. But what if you just want to copy text from an InDesign text frame and paste it into another text frame as plain text (that is, whatever "plain" means in the destination text frame). Thats what CopyAsPlainText does.
To use this script (after you've unzipped it and put the .exe file in your InDesign Scripts folder):
1. Select the text you want to copy.
2. Run the script from the Scripts palette.
3. Click the CopyPlainText button.
4. Click the Text tool where you want to paste the text.
5. Run the script again and click the PasteCopiedText button. InDesign should paste the text you copied as plain text.